The use of radiation of various types and energies for charging, for machining workpieces to be machined, or for altering material properties of workpieces to be machined has become widespread in the art for a wide range of fields of application.
In this context, photon radiation (in other words in particular charging with light, charging with X-ray radiation, UV light, infra-red light and the like) is not the only possible type of radiation; in particular, particle radiation may also be considered. In this context, the particles may be substantially as desired (“particles” in this context meaning in particular particles which have a rest mass, even though it may be extremely small). Hadrons and leptons may be mentioned purely by way of example, in particular including neutrinos, electrons, positrons, pions, mesons, protons, neutrons, atomic nuclei (for example He nuclei), atoms or molecules and ions (in particular including heavy ions such as oxygen ions, helium ions, neon ions or carbon ions).
What all these types of radiation have in common is that the radiation deposits a particular energy in the item charged with radiation. However, in some cases the manner in which this energy is deposited varies greatly. Whilst for example in the case of photon radiation the energy loss is related approximately exponentially to the material penetrated over wide energy ranges, particle beams, in this case in particular hadron particles (especially protons, ions and heavy ions), have a pronounced Bragg peak. The particles thus initially lose comparatively little energy on the path thereof upon penetrating material. Shortly before the particles come to rest, the majority of the energy is released into the material charged with the radiation. As a result of this Bragg peak, not only two-dimensionally structured dose charges, but in particular also three-dimensionally structured dose charges can be realised (in other words different deposited radiation doses at different depths in the irradiated object).
Not only may the type of radiation used vary, but so also may the type of objects charged with radiation. To name just a few technical fields of application, possible examples relating to charging with protons in structuring processes include masks and material removal or material application in the manufacture of structured semiconductor components (such as memory elements, microprocessors and the like). Photons may also be used for cutting and/or welding workpieces (in particular if the photon radiation is in the form of a high-energy laser beam).
One example application for electron beams is electron beam welding, by means of which for example two metal workpieces can be welded together. Naturally, separation and structuring processes are also conceivable.
In medicine and veterinary medicine, radiation is used for therapeutic purposes. For example, it is known to use X-ray radiation for producing X-ray images (including three-dimensional images from CT (computed tomography) methods). Electron beams have also been used in medicine for several decades, for example for treating cancerous tumours. Treatment for tumours using protons and ions (in particular heavy ions) has also now become well established in medicine. Because of the previously described Bragg peaks of protons/ions/heavy ions, it is possible to charge a three-dimensionally defined and structured region (in particular a tumour) in a patient with radiation in a targeted manner by controlling a particle beam accordingly (for example as part of a scanning process), whilst the surrounding tissue is largely unaffected. Precisions in the millimeter range are now possible. In this context, in some cases widened particle beams are used, which are structured spatially resolved in terms of the energy thereof (and thus the penetration depth thereof into the tissue), for example using paraffin plates of varying thicknesses. As a rule, however, scanning methods are now generally used in which, by means of suitable deflection magnets and suitable energy variation (leading to a variation in penetration depth), a conventionally thin (pencil-thin) particle beam successively “approaches” the different volume regions to be charged with a dose of the object to be irradiated.
So as to be able to treat three-dimensional volume regions in a body (in particular to treat cancer in a patient), it is usually necessary to prepare a radiation treatment plan. In this context, a particular radiation pattern is computer-simulated (in other words a sequence with different x-y deflections of the particle beam and suitable particle energies of the particle beam) and the respectively resulting dose input into the body charged with the radiation is calculated as a function of location. This is because, although the deposited dose in the irradiated object is concentrated on the region of the Bragg peak, a particular dose is nevertheless deposited in particular in regions lying close to the radiation point along the particle path. In the context of radiation treatment planning, an attempt is made to prepare a radiation treatment plan in which a particular minimum dose is reliably exceeded within a desired target volume region to be irradiated (in such a way that for example cancer cells are reliably destroyed), but surrounding tissue is spared as much as possible, in other words a particular maximum dose is not exceeded. The maximum dose to be set in this context may vary greatly depending on the irradiated object region. If for example in medical applications particularly vital and/or radiation-sensitive tissue regions are present, these have to be spared to the maximum extent. In this context, the term OARs (organs at risk) is generally used. In this context, a particularly suitable radiation treatment plan for the respective use can take place by actuating the particle beam differently and/or by introducing the particle beam from a plurality of different directions (possibly also a number of different directions including pivoting which is continuous at least at times).
Particular problems occur if (sub-regions of) the object to be irradiated move. In this context, movement may include not only translational movements, but also twisting movements and/or compression or extension movements. In particular in combination with scanning methods, the movements of the object and those of the particle beam may “interfere” with one another and lead to comparatively poor radiation results if suitable countermeasures are not taken.
Various solution approaches have been proposed in the art so as to be able to irradiate moving target areas.
For example, a first solution approach involves using corresponding tracking of the particle beam to compensate the movement of the target volume region in the object. However, problems with this method include the greatly increased equipment complexity and the problem that dose deposition effects in object regions outside the actively irradiated irradiation point cannot be predicted in the advance radiation treatment planning. As a result, particular comparatively complex monitoring and compensation methods are necessary. Even determining the actual movement of the target area in the context of the actual irradiation is often found to be problematic or virtually impossible. In particular in medicine, certain types of movement are not accessible or are difficult to access by “tracking” of this type.
Another proposal involves determining all of the possible movement states in advance in the radiation treatment planning (for example by CT methods) and selecting the area irradiated during irradiation to be suitable and sufficiently large so that the region to be irradiated is in any case charged with a particular minimum dose. In this context, some (inherently undesirable) damage of the surrounding tissue is deliberately accepted (or deliberately included in the planning in the context of safety margins). A problem during planning is that in particular in medical applications an (internal) movement of a target volume region in the patient not only leads to a change in the geometric position of the corresponding volume region, but also has an additional effect on the penetration lengths of the particle beam into the tissue, in particular by way of expansion and compressive movements and the accompanying density changes.
A method for taking into account effects of this type has been disclosed for example in German Offenlegungsschrift DE 10 2007 014 723 A1. In this context, a planning target volume is determined in that initially a target volume equivalent to the minimum target volume in the body is determined in an imaginary homogeneous body. The equivalent target volume is expanded by a safety margin to determine the planning target volume. A problem with the method proposed therein is that the conversion to an imaginary homogeneous body is heavily dependent on the direction of incidence of the particle beam. Preparing/optimising a radiation treatment plan in which there is a radiation input from two or more different directions is not possible with the method proposed therein. However, methods of this type using different radiation input directions are desirable so as to reduce the exposure of surrounding tissue (in particular OAR tissue) in so far as possible.
Even though some improvements to the method described therein using suitable transformation and back-transformation algorithms have already been considered, known methods still have problems. In particular if a movement of the target volume region is combined with different irradiation directions, in known methods this may lead locally to deviations from a target dose to be deposited (known as incorrect dosing). Above all, however, “gaps” (local underdosing) in the irradiated area are highly undesirable, since they may make it possible for cancer cells to survive, and thus drastically jeopardise the treatment success.
There is thus still a need for a method for preparing a radiation treatment plan which is improved over methods known in the art. There is likewise a need for devices for preparing a radiation treatment plan which are improved over devices known in the art.